Chromatin undergoes reversible folding in order to allow genes to be transcribed, replicated and separated during mitosis. Histones are involved in the compaction of chromatin and the reversible histone modifications that affect the core histone N-termini and histone H1 have been implicated in these functions. In the yeast, Saccharyomyces cerevisiae histone deletions and point mutations will be made to alter core histone domains and sites of acetylation which may be involved in folding. We propose to obtain conditional mutations in histones and develop and inducible gene system which will allow the results of deleterious mutations to be analyzed biochemically. Assays will be set up utilizing electron microscopy, sedimentation and nuclease hypersensitivity to establish which histone domains and sequences are invloved in folding. The gene for a histone H1-like protein in yeast will be characterized by sequence and genetic analysis and the protein by chromatin reconstitution to determine the function of this protein. Finally, a distantly related yeast, Schizosaccharomyces pombe, will be studied in order to determine the generality of our key findings in S. cerevisiae.